1. Field of the Invention
This invention relates to a method of fabricating a single crystal silicon by the Czochralski method (hereinafter referred as the CZ method), particularly to a method for manufacturing single crystal silicons, which is capable of preventing the occurrence of defects within the portion near the rear part of a single-crystal body.
2. Description of the Related Art
The substrates of semiconductor components are mainly made of high-purity single crystal silicon that is conventionally produced by the CZ method. In the CZ method, polycrystalline silicon lumps are fed into a quartz crucible of a single crystal silicon pulling device. Then the quartz crucible is heated by cylindrical heaters disposed therearound to melt the polycrystalline silicon lumps, thereafter a seed crystal installed on a seed chuck is dipped into the melt of raw material. After that, the seed chuck and the quartz crucible are respectively driven to rotate in opposite or identical directions, and at the same time the seed chuck is pulled to grow a single-crystal silicon ingot of predetermined diameter and length.
In device processes, various heat treatments are applied on single crystal silicons produced by the CZ method. However, as shown by the dotted graph in FIG. 6, there exists a portion near the rear part of a single-crystal body 2 whose diameter is constant, which has markedly greater oxygen precipitation than other portions. In addition, as shown by the solid graph in FIG. 6, there is a shadowed section located near the rear part of the single-crystal body 2 that has the fragment containing anomalous oxygen precipitation (hereinafter referred as AOP) as its boundary. In the above section, there is a high density of grown-in defects such as Lazer Scattering Tomography Defect (hereinafter referred as LSTD), Flow Pattern Defect (hereinafter referred as FPD), and Crystal Originated Particle (hereinafter referred as COP), which greatly affect the breakdown voltage of the oxidation film of the devices, and the distribution of the defects is uneven. The above phenomenon is caused during the cooling process subsequent to the pulling of single crystals. Semiconductor devices made of wafers sliced from a zone closer to the tail portion than the AOP fragment will display greater current leakage and inferior oxidation film voltage-endurance strength, thus severely decreasing the yield. The greater current leakage is induced by excessive oxygen precipitation in the device processes. In other words, the zone closer to the tail than the AOP fragment can not be made into high-quality silicon wafers via the CZ method, and this is one of major causes of the low yield for single crystal silicons.